Urban systems are generally single-user networks, which makes life a lot easier. For main line railways that assumption doesn't hold, which is where things kinda fall apart.
It's a bit like the old copypasta from the slashdot days of "This is why your solution to spam won't work", where one of the options was "It requires the entire internet to change at once.
Just look at what a big deal in the US it was to get positive train control (PTC) installed nationally.. and that's a much simpler system that is basically local (Picks up info from relays in the track, but all the logic is in the locomotive).
It's a bit like the old copypasta from the slashdot days of "This is why your solution to spam won't work", where one of the options was "It requires the entire internet to change at once.
Just look at what a big deal in the US it was to get positive train control (PTC) installed nationally.. and that's a much simpler system that is basically local (Picks up info from relays in the track, but all the logic is in the locomotive).